Most contact lenses that will directly contact the eye require a composition, for example, to store, condition, rinse, or reduce the microbial load on the lens. Most, if not all, of the compositions used for contact lens treatment require an additive with antimicrobial, disinfection, and/or preservative capabilities. The antimicrobial/preservative should be designed to cause minimal ocular irritation or user discomfort since this antimicrobial/preservative will likely come into contact with the eye (either directly via solution carry-over when the contact lens is placed into the eye or via a lens through chemical uptake and subsequent release into the eye.)
Many different microorganisms may be found on contact lenses or in lens cases. Stenotrophomonas maltophilia is a gram-negative bacterium which is the second most common gram-negative general clinical isolate after Pseudomonas aeruginosa. S. maltophilia is emerging as an important ocular pathogen: 1/1339 ocular specimens were S. maltophilia in the 1970s, 1/413 in the 1980s, and 1/363 in the 1990s through 1995. Stenotrophomonas is not an uncommon cause of microbial keratitis, infiltrative keratitis, and contact lens acute red eye among lens wearers. S. maltophilia enhances biofilm formation by P. aeruginosa and likely P. fluorescens and in so doing may facilitate other infections. S. maltophilia has high binding to and internalization into Acanthamoeba and has been reported to have the second highest growth enhancement of Acanthamoeba. It is known to be an important food source for Acanthamoeba species and thought to contribute to Acanthamoeba keratitis among contact lens wearers for this reason. In this context, S. maltophilia has been isolated as a co-contaminant of contact lenses and solutions, along with Acanthamoeba. In 2006, combined Acanthamoeba and S. maltophilia keratitis was reported. Delftia acidovorans is another gram-negative bacterium which is also an ocular pathogen, although reported to occur less frequently in ocular infections than Stenotrophomonas. D. acidovorans has also been reported as a food source for Acanthamoeba. S. maltophilia and D. acidovorans are among the top 3 most frequently isolated gram-negative bacteria in contact lens cases. Contact lens storage case contamination is associated with contact lens contamination, which leads to ocular infection. Importantly, it can be difficult for some contact lens cleaning solutions to satisfactorily disinfect against S. maltophilia and D. acidovorans and thus prevent infections arising from these organisms.
Many multi-purpose solutions (MPSs) that may be used to clean, disinfect, and wet contact lenses, followed by direct insertion into the eye, are available. Multi-purpose solutions must be strong enough to kill harmful microorganisms that may be present or grow on the lenses while being gentle enough to contact the eyes. Such a solution also must be compatible with the many contact lens materials, including the silicone hydrogel materials. Measures of contact lens compatibility include contact lens discoloration, physical parameter change, fragility, and uptake/release of solution components, especially antimicrobial agents. Contact lens care solutions, such as MPSs, attempt to balance cleaning and disinfection ability with safety and comfort on the eyes. The addition of more effective disinfecting agents usually has the effect of reducing contact lens material compatibility or ocular comfort of the solution. One way to achieve additional material compatibility and comfort is to reduce the amount of disinfecting agent. However, conventional knowledge dictates that this results in lower antimicrobial efficacy. While many antimicrobials have been developed for ophthalmic use, many of these compounds can be absorbed into lenses themselves, thus decreasing their efficacy. One way to address the absorption issue is to increase the concentration of the antimicrobial. However, this may cause irritation in the eye (particularly when the antimicrobial easily absorbs into the lens). Thus, there is a continual challenge to identify novel ways to improve disinfection without sacrificing user comfort.
One way for a contact lens wearer to increase the biocidal activity of their ophthalmic solution is to use that solution in combination with a silver-coated or silver-impregnated lens case (see, for example, U.S. Pat. No. 5,320,843, to Raheja, et al.). However, such a combination does not always provide increased activity against all organisms, and can even be unreliable against individual organisms (see for example, Dantam et al., Biocidal Efficacy of Silver Impregnated Contact Lens Storage Cases In Vitro, IVOS, 2010.) This article discusses the varying efficacy of three different lens cases against different microorganisms and notes that the lens cases, which the author noted as using broadly similar silver impregnation technology, provide inconsistent biocidal activity against various microorganisms, and minimal if any biocidal activity against specific microorganisms.
There is need for a contact lens cleaning system and method which consistently provides increased biocidal efficacy without causing increased ocular irritation or user discomfort. The disclosed systems and methods address this need by providing synergistic activity against microorganisms which are known to impact contact lens wearers.